Group A streptococci infect through the upper respiratory tract mucosal surface and cause human pharyngitis ("Strep throat"), by first subverting clearance by the respiratory tract mucus and then attaching to the pharyngeal cell. On the molecular level, neither one of these initial events is particularly well understood. Furthermore, very little is known about the factors that influence host susceptibility to such infections. We propose to address this gap by studying one of the earliest interactions between streptococci and humans: the interaction with upper respiratory tract mucus (i.e. saliva). We have previously shown that streptococci specifically bind to the monosaccharide, sialic acid, in commercial available preparations of salivary mucin (the major glycoprotein component of mucus gels), and that the binding to sialic acid is also important in adherence to pharyngeal epithelial cells. Although the upper respiratory tract mucus layer is the site of initial interactions between streptococci and its host, salivary mucus composition has not yet been studied as a factor that influences infection susceptibility. As the first aim of this application, we will determine the concentration of sialic acid in salivary samples from children who exhibit different susceptibilities to streptococcal pharyngitis to determine if a correlation exists between infection rate and salivary composition. We predict that sialic acid concentrations will vary between susceptibility groups, and hypothesize that saliva from individuals who are susceptible to streptococcal carriage or acute infection will contain a higher concentration of sialic acid than non-susceptible individuals. The formulation of this hypothesis is based on previous findings that secretory mucins that coat the upper respiratory mucosa are structurally similar to the surface exposed glycoproteins on underlying tissue. Thus, increased concentrations of sialic acid in salivary mucus will likely reflect an increased number of sialylated receptors on the pharyngeal/tonsillar cells, and this increase in binding epitopes would allow for more efficient colonization of the target tissue. As infection is a dynamic interaction between host and pathogen, it is also important to study how streptococci sense and subsequently react to different host environments. Thus, as a second aim, we propose to examine the transcriptional regulation and gene expression patterns during contact with salivary samples from children with different infection susceptibilities. There are many complex interactions and many potential physiological factors that will play roles in pathogenesis;however little is known about the specific molecular events that occur to streptococci during the interaction with salivary mucus from different children. We predict that the saliva-mediated transcriptome will differ depending on host susceptibility. We hypothesize that genes encoding particular virulence determinants will be upregulated during contact with saliva from infection-susceptible children as compared to saliva from non-carrier individuals. These studies in total may provide initial clues into the reason why certain individuals are more likely than others to develop streptococcal pharyngitis. PUBLIC HEALTH RELEVANCE: One of the long-standing questions regarding streptococcal pharyngitis ("Strep throat") focuses on the reasons why particular children are more susceptible to developing such infections as compared to others. Our study addresses this question from a new perspective and is designed to determine: 1) if increased concentrations of sialic acid in a child's saliva correlates with increased susceptibility to acute pharyngitis or to chronic colonization by streptococci and 2) if contact with saliva from acutely infected children increases streptococcal virulence gene expression. As it estimated that over 600 million cases of pharyngitis occur worldwide annually, and that 3 million children currently suffer from rheumatic heart disease as a result of improper treatment of these infections, identifying factors that increase or decrease a child's susceptibility is an urgent matter and must be addressed if better preventative therapies are to be developed.